1. Field of the Invention
The present invention relates to a process for continuously producing an aromatic carboxylic acid by an oxidation of a poly-alkyl aromatic hydrocarbon in the presence of a catalyst and a reaction accelerator. More particularly, the present invention pertains to the foregoing process which comprises preventing the performance of a condenser from being inhibited by the adhesion to the condenser, of slight amounts of a reaction product and/or a catalytic component that are splashed by accompanying an oxidative reaction exhaust gas which is continuously withdrawn from an oxidation reactor.
2. Description of the Related Arts
An aromatic carboxylic acid is produced by a liquid-phase oxidation of a poly-alkyl aromatic hydrocarbon through an exothermic reaction which is put into practice in a tank type reactor usually equipped with an agitator. In the oxidation process, an oxidation reactor is continuously charged with a poly-alkyl aromatic hydrocarbon, catalytic and components acetic acid containing water as a solvent which is freshly fed or recirculatingly used, each in individual form or as a mixture. The reactor is further charged with a gas containing molecular oxygen at the bottom or in the vicinity thereof.
The aromatic carboxylic acid thus produced is continuously taken out in the form of a slurry solution through a reaction product takeoff port on account of its generally low solubility in acetic acid containing water. After additional oxidative reaction to be conducted at need, the resultant aromatic carboxylic acid is crystallized and separated in one or two or more crystallizers that are connected in series, and is sent to a drying step to be made into the objective product.
The mother liquor separated from the aromatic carboxylic acid, which contains acetic acid containing water as the solvent and soluble catalytic components, is taken out in part to the outside of the reaction system, but the remainder is circulated through the oxidation reactor for reutilization.
The oxidative reaction exhaust gas usually contains at most approximately 8% by volume of oxygen and a small amount of oxidative reaction byproducts each having a low boiling point, and is continuously exhausted through a takeoff port located at the top of the reactor or in the vicinity thereof, while the heat of oxidative reaction is removed by the evaporation of the acetic acid containing water as the solvent, and the evaporated acetic acid containing water accompanies the oxidative reaction exhaust gas, and is exhausted therewith.
The oxidative reaction exhaust gas which contains the evaporated acetic acid containing water is condensed by being passed through one or two or more condensers, and is separated into acetic acid containing water and oxidative reaction exhaust gas composed of non-condensable components. The resultant oxidative reaction exhaust gas is exhausted to the outside of the reaction system directly or at need, through an valuable component recovery step and/or an energy recovery step.
Part of the acetic acid containing water thus separated is sent to a dehydration step, where the water formed by the oxidative reaction is removed, while the dehydrated acetic acid is reused as a solvent for oxidative reaction. The remainder of the acetic acid containing water thus separated is directly returned to the reactor, where it is reused as a solvent for oxidative reaction.
A gas containing molecular oxygen is continuously supplied to the bottom or in the vicinity thereof of the oxidation reactor, where the gas rises through the liquid therein, causing vigorous stirring so as to proceed with the oxidative reaction of a poly-alkyl aromatic hydrocarbon.
The oxidation process for a poly-alkyl aromatic hydrocarbon as mentioned hereinbefore is characterized by markedly high rate of reaction, and has been employed for many years as a commercial process for the production of an aromatic carboxylic acid.
One of the problems with the above-mentioned process for producing an aromatic carboxylic acid consists in solid matters accompanying the oxidative reaction exhaust gas which is exhausted at the top of the reactor or in the vicinity thereof. The foregoing solid matters include not only fine solids of aromatic carboxylic acids, reaction byproducts and intermediate products each accompanying the oxidative reaction exhaust gas, but also aromatic carboxylic acids, reaction byproducts, intermediate products, catalytic components and the like each being dissolved in the droplets that are accompanied thereby.
The solid matters accompanying the oxidative reaction exhaust gas involve danger of adhering to walls of piping and inside walls of machinery and equipment on the downstream side of gas stream, and thereby impairing smooth running of reaction equipment. What is particularly problematic is the danger that the solid matters adhere to walls of a condenser which is placed immediately close to the downstream side of the reactor. When the solid matters, even if in a small amount, adhere to the walls of such a condenser, heat transfer is markedly inhibited, thus impairing the normal function of the condenser. If the worst case should happen, the gas stream itself is inhibited by blocking due to accumulated solid matters.
In such circumstances, various countermeasures thereaginst are proposed to suppress to the utmost, the foregoing solid matters accompanying the oxidative reaction exhaust gas. The proposals include, for instance, a method which comprises preserving a relatively large space above the liquid surface in a reactor, a method which comprises installing some shields or the like between vigorously bubbling liquid surface and a takeoff port of the oxidative reaction exhaust gas, a method which comprises imparting cleaning effect against solid matters by contriving and modifying the mechanism of returning to the reactor, acetic acid containing water as solvent that has been condensed, and the like methods.
Even if any or all the above-mentioned methods are devised, it is still impossible to perfectly eliminate the problematic solid matters accompanying the oxidative reaction exhaust gas.
It is a general object of the invention to provide a method for almost perfectly eliminate the occurrence of troubles and obstructions in a condenser which is caused by the solid matters accompanying the oxidative reaction exhaust gas in a process for producing an aromatic carboxylic acid by the oxidation of a poly-alkyl aromatic hydrocarbon.
Other objects of the invention will be obvious from the text of the specification hereinafter disclosed.
As a result of intensive research and investigation carried out by the present inventors, it has been found it possible to prevent the aforesaid troubles and obstructions in a condenser due to the solid matters accompanying the oxidative reaction exhaust gas by supplying part of a solvent to an inlet of oxidative reaction exhaust gas leading to a condenser. The present invention has been accomplished by the above-mentioned findings and information.
Specifically, the present invention provides a process for continuously producing an aromatic carboxylic acid which comprises
(A) a step of subjecting a poly-alkyl aromatic hydrocarbon to a liquid-phase oxidation by continuously supplying an oxidation reactor with said hydrocarbon, a catalyst, a reaction accelerator, acetic acid containing water and a gas containing molecular oxygen gas;
(B) a step of continuously taking out from the oxidation reactor, the reaction products containing aromatic carboxylic acids, and separating the same into crude aromatic carboxylic acids and acetic acid containing water; and
(C) a step of continuously taking out from the oxidation reactor, the oxidative reaction exhaust gas containing evaporated acetic acid containing water, introducing said gas in a condenser, and condensing the evaporated acetic acid containing water,
wherein the step (C) further comprises supplying acetic acid containing water to an oxidative reaction exhaust gas inlet line leading to the condenser.